The analytical system is designed to ensure a reduced sample requirement in the microliater-range, low absolute detection limits up to below the pg-range, a high dynamic range of the analytical signal, and small trace determination errors due to the main constituents of the sample.
It is known that mass spectrometers can be used for the elemental analysis of solid samples by application of a spark ionization MS, or a glow discharge in connection with an MS, U.S. Pat. No. 4,794,252, and that the analysis of liquid and aerosol samples by means of inductively coupled plasma, ICP-MS, is possible. Spark mass spectrometers and glow discharge mass spectrometers are expensive and time-consuming systems and not suitable for the analysis of liquid or aerosol samples. An ICP-MS requires, because of the ICP which is operated at atmospheric pressure, high pumping capacities for a differential pumping system in order to maintain a high vacuum in the MS. Both the introduction of the sample through an atomizing system and the transfer of the ions from the ICP to the MS are highly inefficient, and this restricts the detection sensitivity and requires relatively high sample quantities in the ml-range.
The introduction of dried aerosols of the analytical sample in an MS is known, U.S. Pat. No. 4,403,147. Such systems require a carrier gas stream for the transport of the aerosol, and thus also high pumping capacities, similar to the ICP-MS. The atomization and ionization of the aerosol causes high additional cost in order to achieve an acceptable efficiency.
In usual quadrupole mass spectrometers, i.e. in ICP-MS at present, only ions of a certain ionic mass are recorded at any time, whereas the ions of all the other masses are lost. The same applies also to sector-field mass spectrometers with a single exit slit. In both cases, only 0.1% or 0.01% of the ions which have been formed at one time, contribute typically to the intensity recorded in the spectrum. However, if it is possible to store for a while all the ions formed over a longer period in a storage unit prior to the mass analysis, and to use a mass spectrometer with simultaneous mass detection, all or at least a very high percentage of all the ions which have been formed at one time are also detected.
It is known from atomic absorption spectrometry that extremely high atomization efficiencies can be achieved with electrothermal atomizers.(H.F. Falk, CRC Critical Reviews 19, Issue 1, p. 29-64, New York, 1988). These atomizers are operated, however, at atmospheric pressure and not suitable in this form for coupling to an MS.
Various realizations of ion traps based both on electromagnetic and electrostatic effects are known. Furthermore, combinations of time-of-flight mass spectrometers and ion traps have also been realized. ("Rapid Communications in Mass Spectrometry, 2, 1988, 83-85). These systems are, however, not suitable because of their design for the analysis of solid, liquid or aerosol samples.
In ion traps, an appropriate control system makes it possible to use the trap first as storage unit, and subsequently as mass analytical system. If a separate storage unit is provided, the ion can be injected after extraction into any mass spectrometer, i.e. also into a quadrupole spectrometer or a sector-field mass analyzer. A further advantage of time-of-flight mass spectrometers is the possibility of ensuring that the ions of various masses start approximately simultaneously (or after exactly defined mass-dependent periods). This "simultaneous" start can be achieved by concentrating the period in which the stored ions are released (about 1 microsec) after an acceleration path to considerably shorter times (about 10 nsec), similar to the klystron bunching. This results, however, in a substantial increase of the energy width of the ion distribution, so that the time-of-flight mass spectrometer has to be relatively highly energy isochronous. This means that the ions of a mass, which have slightly varying energies, have to feature the same time of flight, this being achieved by ensuring that the higher energy ions can reach the detector only via an appropriately designed detour, e.g. by providing an ion reflector, into which the higher energy ions penetrate to a greater depth.